1. Field of the Invention
The present invention involves a fiber collection system and more particularly a collection system for collecting very small diameter staple fibers such as fibers having an average diameter of less than 1.5-2 microns. The collection system disclosed, and the method of using it, is particularly effective for collecting fibers having a mean diameter of less than one micron such as glass microfibers used to make high efficiency particle arresting filters for cleanrooms. The fiber product collected is believed to contain a higher percentage of smaller diameter fibers than have been present in fiber products heretofore.
2. Discussion of the Prior Art
A known system and method for collecting small diameter fibers is disclosed in U.S. Pat. No. 4,167,404, the disclosure of which is hereby incorporated by reference. The system disclosed in this reference (Loeffler system) has been representative of the state of art in this technology for many years until the present invention. In the system described in this patent, minute fibers having diameters from a few hundredths of a micron to many microns, but averaging about 0.1 to about 2 microns, suspended in a rapidly moving hot gaseous stream containing products of combustion and air are largely removed from the gaseous stream on a woven wire screen on the surface of a rotating permeable drum.
While the Loeffler system has functioned well for many years it still has some limitations and short comings. It was designed to operate with pressure drops of less than one inch of water and actually was operated most efficiently at pressure drops of about one-half an inch of water pressure. One of the reasons for this limitation is that the interior seals on the collection drum won't hold up and leak as the drum tends to go out of round due to the constant heating and cooling. Also the exterior seal that rides across the screen on the collection drum leaks due to the uneven nature of a woven screen surface reducing further the pressure drop across the collection surface of the drum.
Another undesirable feature of the Loeffler system is that the fiber is very hot as it is removed from the collection drum, as high as 300 degrees Fahrenheit. This is too hot to handle or to package in plastic bags and has to be handled with insulating gloves or mechanically until it has cooled. Cooling takes considerable time since the compressed fiber has excellent thermal insulating properties.
Other undesirable features of the Loeffler system are the maintenance required to occasionally replace the screen used on the collection surface of the drum and the need to water cool the drum axle bearing that sets in the hot exhaust air to achieve decent bearing life. Also, the method of removing the fibers from the collection drum by winding on a mandrel makes closing up the system very difficult and expensive, and it is desirable to close up the system to prevent external contamination and improve the working environment. Finally, the ever increasing cost of energy and standards for exhaust air cleanliness from manufacturing processes create a need for a system having still higher collection efficiency and lower volumes of exhaust air per unit of product produced.